1. Field of the Invention
The present invention relates to a lithium secondary cell having metallic lithium or an alloy thereof as an active anode substance and a method for manufacturing same, and more particularly to a lithium secondary cell and method for manufacturing same, which does not have a tendency for dendrite growth, and which has superior cycle characteristics.
2. Related Art
Although a lithium cell with a anhydrous electrolytic fluid using metallic lithium features superior energy density and a high electromotive force, when charged, it exhibits dendrite formation on the lithium surface of the anode, and in some cases, the dendrite formation can break through a separator, causing a short to the cathode or peeling of the lithium dendrite from the conductive base (collector). These factors cannot only cause the cell not to function, but can also lower the safeness of the cell or worsen the cycle characteristics of the cell, due to abnormal reactions caused by shorting.
In order to suppress dendrite growth in metallic lithium, it is desirable to perform dielectric reactions having uniform precipitation and reaction speed. For this reason, it is necessary that the metallic lithium crystal structure be uniform, and that the surface reaction and coverage on the surface at which the reaction occurs be uniform.
Given the above, there have been proposals, for the purpose of suppressing dendrite formation, of forming alloys of metallic lithium with components of aluminum, bismuth, lead, tin, or indium or the like, or of forming an oxide layer on the surface of the metallic lithium. Using these methods, however, the operating voltage of the cell is low compared to the case of using an anode made of metallic lithium, and there is a reduction in the energy density. Additionally, when such alloys are used repeated charging and discharging results in deterioration of the lithium alloy in the form of micro-powder formation.
Another proposal that has been made is that of forming a covering film of lithium fluoride or the like, making use of a chemical reaction on the surface thereof, so as to suppress dendrite formation.
For example, in the Japanese Un-Examined Patent Publication (KOKAI) No. 7-302617, there is disclosure of an anhydrous electrolytic cell having a covering including a lithium compound such as lithium fluoride on the surface of the anode. This lithium fluoride covering film is obtained exposing a anode having a covering film or lithium hydroxide or lithium oxide formed by natural oxidation in the atmosphere to an electrolytic fluid including LiPF6, to cause a reaction between the lithium fluoride and water, and then causing a reaction with lithium hydroxide. Because these lithium fluoride-covering films make use of a reaction with water at an electrode boundary, however, there is a tendency for components from sub-reactions intruding into the covering film, thereby making it difficult to achieve a uniform covering film. There are also cases in which the lithium hydroxide or lithium oxide covering film is not form uniformly, or in which there are locations at which lithium is partially exposed, in which cases not only is it impossible to achieve formation of a uniform thin film, but also there is a problem with safety, because of an abnormal reaction with water or hydrogen fluoride or the like. If the reaction is insufficient, there will be a residue of an unwanted covering film other than the fluoride, leading to an adverse effect, such as a reduction in ion conductivity.
Additionally, with a method such as this, in which a fluoride layer is obtained by making use of a chemical reaction at a boundary, there is a limitation on the possible fluorides and electrolytic fluids that can be used, thereby making it difficult to achieve a stable covering film. For this reason, this method does not achieve a sufficient cycle life, and has still not been developed to a practically usable stage.
In the Japanese Un-Examined Patent Publication (KOKAI) No. 8-250108, there is disclosure of method for forming a lithium fluoride covering film, by placing a metallic foil made of metallic lithium or a lithium-aluminum alloy in an atmosphere of argon gas or nitrogen gas containing a small amount of hydrogen fluoride so as to cause a reaction, thereby forming a lithium covering film on the surface of the metallic foil (anode). In this case, if a covering film already exists on the surface of the metallic lithium, there is a tendency for the reaction to be non-uniform, particularly if there is a plurality of covering films, thereby making it difficult to form a complete lithium fluoride covering film. Additionally, because this method uses a reaction between hydrogen fluoride and a boundary with metallic lithium to form lithium fluoride, it is not suited to the case in which it is necessary to form a thick lithium fluoride covering film. As a result, it is not possible to achieve a lithium secondary cell with satisfactory cycle performance.
Additionally, in Japanese Un-Examined Patent Publication (KOKAI) No. 11-288706, there is disclosure of improvement in the cycle life of a battery, by forming a surface covering film having as a main component a rock-salt type crystal structure, on the surface of s lithium sheet oriented with preference to a uniform crystal structure (that is, 100), thereby enabling suppressing a dendrite precipitation reaction of the metallic lithium. This disclosure mentions that it is preferable that the substance used as a surface covering film be selected as at least one lithium halide, selected from among LiCl, LiBr, and LiI. In this case, in order to form a solid solution covering film of at least one of LiCl, LiBr, and LiI and LiF, a lithium sheet formed by pressuring (cold rolling) and oriented preferably for (100) on the crystal plane is immersed in a dielectric fluid containing at least one of a sodium molecule or sodium ion, a bromine molecule or bromine ion, and an iodine molecule or iodine ion with a fluoride molecule or fluoride ion. This technology makes use of a cold-rolled lithium sheet, and because it is easy to expose the lithium sheet to the atmosphere, there is a tendency for a covering film to be formed on the surface due to water content and the like, leading to non-uniformity in the activated points, making it impossible to achieve the intended stable covering film and causing insufficient suppression of dendrite growth. Additionally, because of the immersion into an electrolytic fluid, the same problems arise as noted with regard to the Japanese Un-Examined Patent Publication (KOKAI) No. 7-302617. Additionally, because of the use of a cold-rolled lithium sheet, compared with the case of using an amorphous lithium layer, there is a greater tendency toward dendrite formation, leading to possible insufficient cycle life.
In the Japanese Un-Examined Patent Publication (KOKAI) No. 7-296812, there is a proposal for using, in place of using a film fabricated by cold rolling, a anode having minimally at its surface a layer made of amorphous lithium or an alloy of amorphous lithium. An amorphous form is used in this technology to impede the formation of activated points at the crystal grain boundaries, which are characteristic points for dendrite growth on the surface of a metallic lithium anode, thereby achieving a uniform precipitation of lithium ions. Furthermore lithium precipitated by virtue of a high electron hole density, based on an amorphous structure is efficiently dispersed within the anode, thereby preventing the occurrence of concentrations of activated points at specific locations on the surface of the anode. It is difficult, however, using just an amorphous structure alone, to achieve a battery with good cycle performance. Additionally, with only amorphous lithium or an amorphous lithium alloy, there is a tendency for a reaction to occur at the time of fabricating the battery with water contained in the atmosphere, the electrolytic fluid, the cathode, or the separator or the like, making it necessary to establish strict conditions for the constituent elements battery fabrication process and the constituent elements thereof.
Accordingly, it is an object of the present invention to provide a lithium secondary cell featuring suppressed dendrite growth in a anode, which contains a metallic lithium or an alloy thereof, and featuring superior energy density, superior electromotive force, and superior cycle life.